Thermoplastic cellular network toughened composites

ABSTRACT

A composite article including fiber tows and a network including material drawn or pulled between the fiber tows. The network forms a physical barrier reducing propagation of cracks in the composite article. Exemplary structures described herein are the first to use a novel cellular architecture to toughen resin infused composites and create a continuous through thickness reinforcement that does not induce fiber breakage.

BACKGROUND 1. Field

The present disclosure describes novel composite structures and methodsof fabricating the same.

2. Description of the Related Art

Composites are replacing metals as structural materials because of theirlight weight, relative strength, and their ability to be molded intomore complex shapes. However, conventional composite structures canexhibit cracking (in particular delaminations) under stress.Conventional composite toughening techniques in the form of interlayers(such as thermoplastic veils and particles) exhibit limitedeffectiveness in controlling failure at high stress concentrationregions. In many cases, a toughened film adhesive is required to achievethe required through thickness toughness properties. Moreover,mechanical fasteners may be the default for acceptable design. This isparticularly a concern for integrated aircraft structures where areas ofhigh stress (100) occur throughout the structure thickness such as at ajoint radius (see FIG. 1). Therefore, a crack (102) that develops at thejoint radius would only need to jump to an untoughened layer for brittlefailure to occur. Such cracking might be mitigated using a throughthickness toughening technique. However, conventional through thicknessmethods (such as film adhesives) are not employed in liquid moldedstructures due to disruption of the resin flow path during infusion,leading to defects such as voids, porosity and dry spots. Such defectssignificantly lower in-plane properties of the composite laminatestructure.

Examples of through thickness techniques and their constraints arelisted below:

-   -   Z-pins: used for stacked reinforcements but are not currently        used due to microstructural imperfections that occur during the        insertion process.    -   Stitches: used for toughening dry fiber preforms but are not        currently used due to microstructural imperfections that occur        during the insertion process.    -   Three dimensional (3D) woven/3D knitted/3D braided preforms that        are typically applied with liquid molding methods. However, due        to fiber misalignment within the 3D preform caused during the        manufacturing processes, these preforms are limited to specific        geometries and are not readily applied at integrated aircraft        joints.

FIG. 2 shows an Ashby plot characterizing conventional veil, stitch andz-pinned toughened composites and highlights the gap where in planeproperties have been measured as Open Hole Compression (OHC) strengthversus Mode I Interlaminar Fracture Toughness (Gic). FIG. 2 shows thatthrough thickness methods significantly increase mode I fracturetoughness compared to veils and that an increase up to 1400% is possiblewith z-pins. However, through thickness reinforcements reduce damagetolerance in terms of OHC strength and other in-plane properties due tothe microstructural imperfections induced during their manufacturingprocess. Veils applied as interlayers, on the other hand, show a lowerreduction in OHC, where less disruption to the fibers is caused duringthe application of veil to the fiber preform. However, the effectiveimprovement in Gic is poor. Thus, the gap representing performance thathas not been conventionally achieved is obtaining a high mode I fracturetoughness without degradation of damage tolerance and in-planeproperties using continuous through thickness reinforcements.

What is needed, then, is a through thickness technique that iscontinuous through the composite and does not reduce in-planeproperties. The present invention satisfies this need.

SUMMARY

The present disclosure describes a composite article (300) including aplurality of fiber tows (302) and a network (304) of material (314)combined with the fiber tows (302). The network (304) comprises layers(306 a, 306 b) connected by pillars (308), wherein each of a pluralityof the pillars (308) are drawn from one of the layers (306 a) and passthrough a different space (310) between the fiber tows (302) so as toconnect the one of the layers (306 a) to another of the layers (306 b).The network (304) forms a physical barrier reducing propagation ofcracks in the composite article (300).

The composite article may be embodied in many ways. Examples, includebut are not limited to, one or any combination of the followingexamples.

1. The composite article including a plurality of plies (312), whereinthe plurality of plies (312) each include a plurality of the fiber tows(302) and a plurality of the different spaces (310), wherein at leastone of the plies (312) is between two of the layers (306 a, (306 b)connected by the pillars (308), and the plurality of the pillars (308)drawn from the one of the layers (306 a) pass through the differentspaces (310) in the at least one ply (312) between the two of the layers(306 a, 306 b).

2. The composite article (300), wherein the pillars (308) and/or thelayers (306 a, 306 b) comprise material (314) used for additivemanufacturing.

3. The composite article (300), wherein the layers (306 a, 306 b) and/orthe pillars comprise a thermoplastic or a hybrid of the thermoplastic,

4. The composite article (300), wherein the pillars (308) and/or thelayers comprise a thermoplastic comprising polyamide, polyetherketone(PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK),polyimide (PI), or polyetherimide (PEI)

5. The composite article (300) wherein the fiber tows (302) comprise atleast one material (314) chosen from fiberglass, kevlar, thermoplastic,and carbon.

6. The composite article (300) wherein the pillars (308) and/or thelayers comprise a hybrid of a thermoplastic including at least oneadditive or inclusion selected from a surfactant, a stabilizer, apowder, a fiber, and a particulate.

7. The composite article (300) wherein the fiber tows (302) each have adiameter of at least 2 mm and include at least 1000 fibers, and/or theplies (312) each have a thickness in a range of 2-10 mm, and/or thepillars (308) and the layers (306 a, 306 b) each have a thickness in arange of 2-5 mm, and/or the pillars (308) each have a length in a rangeof 1-3 mm, and/or the composite article (300) has a total thickness in arange of 1.0 mm-50-mm.

8. The composite article (300) wherein the plies (312) comprise thefiber tows (302) arranged into a braided fabric, a woven fabric, anon-crimp fabric, or unidirectional tape.

9. The composite article (300) further comprising resin filling gapsbetween the plies (312) and bonded to pillars (308), the layers (306 a,306 b), or the layers (306 a, 306 b) and the pillars (308).

10. The composite article (300) wherein the layers and/or the pillars(308) have a roughened surface that aids mechanical interlocking withresin.

11. The composite article (300) wherein the pillars (308) are thicker ata base (318) from which the pillar is drawn from the one of the layers(306 a).

12. The composite article (300) wherein the pillars (308) are inclinedfrom the one of the layers (306 a) to the another of the layers (306 b).

13. A joint (1002) comprising the composite article (300).

The present disclosure further describes an integrated aircraftstructure (1000), comprising a skin (1004), a stiffener (1006), and aninterfacial region (1008) between the skin (1004) and the stiffener(1006), wherein the interfacial region (1008) comprises a compositearticle (300) including fiber tows (302), and a network (304) comprisingmaterial (314) drawn between the fiber tows (302) and forming a physicalbarrier reducing propagation of cracks in the composite article. Theinterfacial region (1008) comprising a portion of the skin (1004), aportion of the stiffener (1006), and/or a layer between the skin (1004)and the stiffener (1006).

The present disclosure further describes a method of manufacturing acomposite article (300), comprising (a) depositing material (314) froman outlet (450) onto a base layer (408) while moving the outlet (450)and the base layer (408) relative to one another, first in an x-y plane(412) and then in a z-direction (414), so as to form an anchor (316) onthe base layer (408); (b) moving the outlet (450) and the base layer(408) relative to one another with no feed of the material (314) fromthe outlet (450), so that a portion of the anchor (316) is drawn tocreate a pillar (308); (c) moving the outlet (450) and the base layer(408) relative to one another so that the outlet (450) is positionedabove a next location (702) on the base layer (408); (d) repeating steps(a)-(c) so as to create a plurality of the pillars (308) on the baselayer (408); (e) providing a ply (312) comprising a plurality of fibertows (302) so that each of a plurality of the pillars (308) pass througha different space (310) between the fiber tows (302); and (f) coupling alayer (306 a) to the pillars (308) so that the pillars (308) passbetween the fiber tows (302) before connecting with the layer (306 b);and so that a composite article (300) comprising the pillars (308), thelayer, and the ply (312) is made.

In one or more examples, the base layer (408) comprises a mat (800) ofthe material (314) deposited using three dimensional printing.Alternatively, the pillars (308) may be manufactured using threedimensional printing (e.g., fused deposition modeling) and the fibertows (302) and the layer (306 a) may be fabricated using one or moremethods different from the three dimensional printing.

The method may further comprise repeating steps (a)-(f) using the layer(306 a) as the base layer (408) in the next step (a).

In one or more examples, the ply (312) is placed after the formation ofthe pillars (308).

In one or more further examples, the ply (312) is placed prior toformation of the pillars (308) so that the portion of each of theanchors (316) is drawn between the fiber tows (302) to create thepillars (308).

In one or more examples, each of a plurality of the pillars (308) arebonded to at least one of the fiber tows (302) as the portion of each ofthe anchors (316) is drawn between the fiber tows (302). In yet furtherexamples, a post-processing step is performed wherein each of aplurality of the pillars (308) are bonded to at least one of the fibertows (302) after the fiber tows (302) and pillars (308) have beendeposited.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates high stress concentration regions in a T-Joint.

FIG. 2 illustrates the gap representing performance (in terms ofachieving fracture toughness and maintaining in-plane properties) thatis not achievable in conventional structures.

FIG. 3A illustrates a cellular network combined with fiber towsaccording to one or more embodiments described herein.

FIG. 3B illustrates material deposited in the cellular network includesan anchor, a pillar or extrusion, and a pillar base or extrusion base.

FIG. 3C illustrates an embodiment wherein spaces are defined by fibertows in different layers.

FIG. 3D is a top view of FIG. 3C.

FIG. 3E illustrates an embodiment wherein a resin is combined with thecomposite.

FIG. 4 illustrates an exemplary three dimensional (3D) printer that canbe used to manufacture the composite articles described herein.

FIG. 5 is a flowchart illustrating a method of fabricating a compositearticle according to one or more embodiments.

FIG. 6 illustrates an exemplary trajectory for the print head duringdeposition of a cellular network, according to one or more embodiments.

FIG. 7A illustrates pillars drawn from an anchor using print conditionsof filament feed rate R=0.5 revolutions per minute and nozzle speedF=500 mm/min have a diameter of 150 micrometers, a length of 2.9 mm, andan areal density of 6 g/m².

FIG. 7B illustrates pillars printed on a porous plain weave fiber mat sothat when carbon fiber mats are stacked together, at least a portion ofthe pillars pass through the spaces or pores between the tows.

FIG. 7C illustrates deposition of the cellular network including pillarsand FIG. 7D is a close up view of the cellular network in FIG. 7C.

FIGS. 8A-8D illustrates a process flow for fabricating a compositearticle including vacuum forming.

FIG. 9 illustrates using a hot rolling iron to apply pressure and heatthat encourages movement of the thermoplastic in the cellular networkthrough the fiber layers.

FIG. 10 illustrates an example T-joint incorporating a composite articleas described herein.

FIG. 11 illustrates a processing environment for controlling a 3Dprinter according to embodiments described herein.

DESCRIPTION

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which is shown, by way ofillustration, several embodiments. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present disclosure.

Technical Description I. Example Structures

The current solution for toughening of resin infused manufacturedcomposites is using a thermoplastic veil. Through thickness methods arenot typically employed due to manufacturing limitations and/ormicrostructural defects induced during the manufacturing process.

The toughening method and structures presented herein, on the otherhand, form a novel 3D architecture that can be used within a compositeto enhance toughness by causing cracks to deflect as they encounter cellwalls. In some embodiments, the toughening structures comprisestructures found in bio-composites such as wood, bone, horns and hooves.

FIG. 3A illustrates an example composite article (300) comprising aplurality of the fiber tows (302) intermingled or combined with anetwork (304) comprising layers (306 a, 306 b) connected by drawnmaterial (308 b) comprising pillars (308). Each of a plurality of thepillars (308) are drawn from one of the layers (306 a) and pass througha different space (310) between the fiber tows (302) so as to connectthe one of the layers (306 a) to another of the layers (306 b).

The drawn material (308 b) is not limited to pillars (308). In otherembodiments, also illustrated in FIG. 3A, the drawn material (308 b)comprises a wall (308 c) having an appropriate shape and geometry so asto be drawn from one of the layers (306 a) and pass through each of aplurality of different spaces (310) between the fiber tows (302) andconnect the one of the layers (306 a) to another of the layers (306 b).In one or more examples, layers (306 a, 306 b) may also comprise or formwalls (306 c, 306 d).

The network (304) comprising the drawn material (308 b) and layers (306a, 306 b) forms a physical barrier reducing propagation of cracks(interlaminar, intralaminar, and/or translaminar cracks) in thecomposite article (300). In various examples, the drawn material (308 b)is inclined between the layers (306 a, 306 b) so as to form anon-uniform geometry that increases the surface area of the drawnmaterial (308 b) and creates a more tortuous pathway for the cracks.

In one or more examples, the fiber tows (302) separated by spaces (310)are disposed in a plurality of plies (312). Each of the plies (312) arebetween two of the layers (306 a, 306 b) connected by the pillars (308)or walls (308 c) so that the plurality of the pillars (308) or walls(308 c) drawn from one of the layers (306 a) pass through the differentspaces (310) in the ply (312) between two of the layers (306 a, 306 b).

In typical embodiments, the drawn material (308 d) and layers (306 a,and 306 b) of the network (304) are cell walls created using a 3Dprinter; however the process is not 3D printing in the traditional senseas the pillars (308) or walls (308 c) are not created layer by layer.The 3D printer is instead used as a tool to deposit controlled amountsof material (314) onto a fibrous portion (the plies (312)) in the x-yplane and then the tool uses the plasticity of the material (314) topull the material (314) in a vertical direction.

FIG. 3B illustrates an example wherein the tool uses the plasticity ofthe material (314) to pull the material (314) in a vertical directiontoform thin upright strands or pillars (308). Thus, the deposited material(314) is drawn from an anchor (316) and forms a pillar (308) including abase (318) (e.g., extrusion base) and an upright (320) (e.g.,extrusion), the upright (320) having an average diameter Daverage alongthe upright (320) or extrusion and an angle θ with respect to a verticaldirection (322). In one example, material (314) is also fed or depositedduring the pulling to form the pillar (308). The process conditionsduring deposition or pulling of the material (314) may be controlled toobtain various shapes for the pillar (308). In one or more examples, thebase (318) on the anchor (316) is thicker than the upright (320) portionof the pillar (308).

FIGS. 3C and 3D illustrate examples wherein the plies (312) in thecomposite article (300 e) include a plurality or a stack (360) of plies(336 a, 336 b, 336 c, 336 d) or layers (338 a, 338 b, 338 c, 338 d) eachhaving different orientations (370) of (e.g., unidirectional) fiber tows(340 a, 340 b, 340 c, 340 d). In this case, spaces (342) or pores (344)are created through a plurality of the layers (338 a, 338 b, 338 c, 338d) or plies (336 a, 336 b, 336 c, 336 d) and the spaces (342) or pores(344) are bounded, defined, or walled by the fiber tows (340 a, 340 b,340 c, 340 d) in different plies (336 a, 336 b, 336 c, 336 d) ordifferent layers (338 a, 338 b, 338 c, 338 d).

The plies (336 a, 336 b, 336 c, 336 d) that lie in different planes(346) comprise fiber tows (340 a, 340 b, 340 c, 340 d) aligned ororiented at different angles with respect to each other so as to definethe pores (344) or spaces (342). In one example wherein the plies (312)comprise mats (600) (e.g., as illustrated in FIG. 6B), the fiber tows(302) in each pair of adjacent plies (312) are at 90 degrees to oneanother and are woven together. However, in other examples, the fibertows (340 a, 340 b, 340 c, 340 d) can be oriented at any angle (e.g., 45degrees) with respect to each other. In one or more examples, the heightH of the pillar (308) or wall (308 c) is a function of how many layers(338 a, 338 b, 338 c, 338 d) or plies (336 a, 336 b, 336 c, 336 d)define the walls of the pores (344) or spaces (342). In one or moreexamples, the spaces (342) between the fiber tows (340 a, 340 b, 340 c,340 d) are in an intermediate layer (338 b) between the fiber tows (340a, 340 c).

FIG. 3E illustrates a composite article (300 f) including a resin (380)combined with the plies (312) and the drawn material (308 b).

Examples of the material (314) used to fabricate the drawn material (308b) and the layers (306 a, 306 b) include, but are not limited to, amaterial used in additive manufacturing (e.g., a polymer). The polymermay comprise a thermoplastic, such as polyamide, polyetherketone (PEK),polyether ether ketone (PEEK), polyetherketoneketone (PEKK), polyimide(PI), polyetherimide (PEI) polyphenylsulphone, or hybrid forms of theaforementioned thermoplastics with modifiers and/or inclusions such as acarbon nanotube, graphene, a clay modifier, discontinuous fibers,surfactants, stabilizers, powders and particulates.

In exemplary embodiments, the fiber tows (302) comprise bundles offibers. In various examples the fibers comprise at least one materialchosen from fiberglass, kevlar, carbon, and thermoplastic.

In one or more examples, the fiber tows are woven (302) or arranged intoa braided fabric, a woven fabric, or a non-crimp fabric, or fibrousportion. In other embodiments, the fiber tows (302) are arranged inunidirectional tape with slits or gaps (comprising parallel tows withgaps there between), braids, or multi-axial reinforcements.

In one or more examples, pillars (308) link between intermediate layers(306 a and 306 b), to form a connected network around a distribution offiber tows (302). The fiber tows may comprise of fibers or filaments,arranged in a reinforcement form, including braids, wovens, non-crimpfabrics and unidirectional forms. The filaments may be composedprimarily of carbon, glass, and/or aramid. Other filaments, incombination with the aforementioned, may also include polyamide,polyetherimide, polyetherketone, polyetheretherketone,polyetherketoneketone, polyimide, phenoxy and polyphenylsulphone.Multiple layers of reinforcement may be formed with braids, wovens,non-crimp fabrics and unidirectional formats. The location of thepillars (308) within the pore space between fiber tows within thesemultiple layers of reinforcement connected to the interlayers createsthe (e.g., thermoplastic) network (in x, y, and z directions).

II. Example Manufacturing Methods

a. Network Formation

FIG. 4 illustrates an exemplary 3D printer 400 comprising an extrusionnozzle (402), a feeder (404), and a melter (406) for feeding material(314) onto a ply (312) or base layer (408) and into pores or spaces(310) between the fiber tows (302) in the ply (312), and a platform(410) (e.g., print bed or base) for supporting the plies (312) or baselayer (408) while the material (314) is combined with the plies (312).Example printers 400 include, but are not limited to, a desktop FusedDeposition Modeling (FDM) 3D printer. In one or more examples, the 3Dprinter is controlled by software executing a computer program. Theplatform (410) and/or the nozzle (402) are moved so that the nozzle(402) and the plies (312) or base layer (408) can be moved relative toone another in an x-y plane (412) and vertically up or down in az-direction (414).

The 3D printer can be used to control morphology of the depositedmaterial (314) as illustrated in FIG. 3B, so that key elements of thedeposited material (314) include the anchor (316), base (318) andextrusion or upright (320). In one or more examples, the process forcombining the network (304) and the plies (312) proceeds as illustratedin FIG. 5.

Block 500 represents depositing material (314) from an outlet (450) ontoa base layer (408) while moving the outlet and the base layer (408)relative to one another, first in an x-y plane (412) and then in az-direction (414), so as to form an anchor (316) on the ply (312) at afirst location (700) (see FIGS. 7A and 7B).

In one example, the step comprises:

(1) Depositing material from a nozzle (402) onto the base layer (408)while the nozzle (402) moves first in an x-y plane (412) for a firstpredetermined distance (e.g., 3 mm), so as to form the anchor (316) onthe base layer (408). In one or more examples, the anchor (316) isdefined as a substrate, foundation, and/or source for the pillar (308)providing the material (314) for the pillar (308) and/or providingsomething for the pillar (308) or wall (308 c) to stick to once thepillar (308) is formed.

(2) The nozzle (402) printing in the z-direction (414) on the anchor(316) for a second predetermined distance (e.g., 0.5 mm). The nozzle(402) only prints in the z-direction (414) for a short distance toprevent or suppress the pillars (308) or walls (308 c) from slumping andforming a thick base (318).

Block 502 represents moving the outlet and the base layer (408) relativeto one another with or without feed of the material (314) from theoutlet (450), so that a portion of the anchor (316) is drawn to create apillar (308) or wall (308 c). In one example, after step (2) above, step(3) comprises the nozzle (402) pulling or moving up in a z-direction(414) a third predetermined distance (e.g., 5 mm) with feed of materialor with the feed rate turned off (no feed of the material (314) from thenozzle (402)) and using the stringiness from the material (314) in theanchor (316) to create the uprights (320) forming the pillars (308) inthe z-direction. FIG. 7A illustrates how a portion of the anchor (316)is pulled upwards to create the pillar (308).

Block 504 represents moving the outlet (450) and the base layer (408)relative to one another so that the outlet (450) is positioned above anext location 702 on the base layer (408).

In one or more examples, the step comprises releasing the pillar (308)or wall (308 c) from the outlet (450) (e.g., nozzle (402)) prior tomoving the outlet (450). For example, after step (3) above, thefollowing steps are performed:

(4) With the feed rate of the material (314) from the nozzle (402) on orstill turned off and the nozzle (402) at the third predetermineddistance (e.g., 10 mm) above the base layer (408) or print bed, thenozzle (402) moves to a fourth predetermined distance (e.g., 3 mm) abovea next location (702) on the base layer (408); and

(5) The nozzle (402) head drops down, with the feed rate of the material(314) from the printer (400) on or still turned off.

Block 506 represents repeating steps in Blocks 500-504 (or steps(1)-(5)) so as to create a plurality of the pillars (308) or walls (308c) on the base layer (408).

FIG. 6 illustrates an exemplary trajectory for the print head fordeposition of the pillars, according to one or more embodiments, showingperiods when the nozzle is extruding as the nozzle moves in the x-yplane, periods when the nozzle is not extruding but a structure (e.g.,pillar 308) is formed by moving the print head in a z-direction, andperiods where the nozzle is not extruding and no structure is formed(e.g., between pillars 308). In one or more examples, material (314) isalso fed, deposited, or expelled from the nozzle during periods when thenozzle is pulling or extruding to form the pillar (308).

In one or more examples, the base layer (408) is a ply (312) comprisinga plurality of the fiber tows (302) and a portion of each of the anchors(316) is drawn between the fiber tows (302) to create the plurality ofthe pillars (308) passing through a different space (310) between thefiber tows (302), as illustrated in FIG. 7B. FIG. 7B further illustratesan example where the ply (312) comprises a mat (750). The mat (750)comprises orthogonal (e.g., carbon) fiber tows (752, 754) that are woventogether. The boundaries (756) of the fiber tows (752, 754) define thespaces (310). The base (318) and anchor (316) of each pillar (308) isanchored or attached to the boundaries (756) of the fiber tows (752,754) so that the pillars (308) can pass through the spaces (310).

FIG. 7C illustrates another example wherein the base layer (408) is amat (706) comprising the material (314) or a layer (306 a) of thematerial (314) comprising a cellular network (704). The pillars (308)are then drawn from the mat. In this case, or in other cases where thepillars are pre-formed on a base layer (408) different from a ply (312),the ply (312) is then positioned after the formation of the pillars(308) so that the pillars pass through the spaces (310) between thefiber tows (302).

In one or more further examples, each of the plurality of the pillars(308) or walls (308 c) are bonded to at least one of the fiber tows(302) as the portion of each of the anchors is drawn between the fibertows (302).

In one or more examples, during the step of Block 506, material (314)from the anchor (316) is drawn to the next location (702) (i.e., drawnfrom one pillar (308) to the next pillar (308) being formed), ormaterial (314) is deposited between the pillars (308) so that thematerial (314) forms a plurality of pillars (308) on the base layer(408) and a layer (306 a) connecting the pillars (308).

Block 508 represents optionally coupling a layer (306 a) to the pillars(308) or walls (308 c) (if the layer (306 a) has not been previouslyformed) so that the pillars (308) or walls (308 c) pass between thefiber tows (302) before connecting with the layer (306 a). The step maycomprise depositing material (314) from the nozzle onto the pillars(308) or walls (308 c) so as to form the layer (306 a) connected to thetops of the pillars (308) or walls (308 c) that extend above the fibertows (302). In various examples, the layer (306) comprises a network orweb comprising filaments. In other examples, the step comprisespositioning the layer (306) formed by a different (e.g. non-printed)method.

Block 510 represents optionally repeating the steps of Blocks 500-508 toform a composite article (300) comprising a plurality of layers (306 a,306 b) or walls (306 c, 308 b) and plies (312). For example, after alayer (306 a) of material (314) is deposited or placed on a top side ofa ply (336 d) using the 3D printer (400) so as to connect with thepillars (308) or walls (308 c) (Block 508), a subsequent ply (336 c) isdeposited thereon and the process of Blocks 500-508 is repeated so as toform a stack (360) (referring to the example illustrated in FIG. 3C).Thus, the steps of Blocks 500-508 can be repeated using the layer (306a) (e.g., a mat of the material (314)) as the base layer (408) in thenext step of Block 500.

While the base layer may comprises a mat of the material (314) or ply(312) comprising fiber tows (302) deposited using three dimensionalprinting, in other embodiments, the fiber tows (302) and the layer (306a) and/or mat of the material (314) are fabricated using one or moremethods different from the three dimensional printing.

In some embodiments, pillars (308) or walls (308 c) that do not passthrough the spaces (e.g., pores) are squashed by the stacked plies(312).

Block 512 represents optional post processing steps. The compositearticle (300) may optionally be heated so that the material (314) bondsto the plies (312) after the pillars (308)/walls (308 c) and plies (312)have been formed or deposited. In one or more examples, the material(314) in network (304) (e.g., cellular network (704)) bonds, at variousstrength levels, to the fibers in the fiber tows (302), subject to thesurface tension of the deposited material (314) and melt temperature ofthe material (314). Other postprocessing techniques may be used to heatand bond the fibers in the fiber tows (302) and the cellular network(704) together in 3 dimensions, using vacuum forming or a roller, asshown FIGS. 8A-8D and 9, respectively.

Block 514 represents optionally combining the plies (312) and network(304) with resin (380). The resin (380) may fill gaps between thereinforcement layer (e.g., ply 312) and the cellular network (704). Theresin may bond the cellular network (704) and the plies (312) to form aconsolidated article.

Block 516 represents the end result, a composite article (300)comprising the pillars (308) or walls (308 c), the layer(s) (306 a, 306b), and the one or more plies (312).

The composite article may be embodied in many ways. Examples, includebut are not limited to, one or any combination of the followingexamples.

1. The composite article (300, 300 b, 300 c) including a plurality offiber tows (302) and a network (304) of material (314) combined with thefiber tows (302). The network (304) comprises layers (306 a, 306 b)connected by pillars (308), wherein each of a plurality of the pillars(308) are drawn from one of the layers (306 a) and pass through adifferent space (310) between the fiber tows (302) so as to connect theone of the layers (306 a) to another of the layers (306 b). The network(304) forms a physical barrier reducing propagation of cracks in thecomposite article (300).

2. The composite article (300, 300 b) including a plurality of plies(312), wherein the plurality of plies (312) each include a plurality ofthe fiber tows (302) and a plurality of the different spaces (310),wherein at least one of the plies (312) is between two of the layers(306 a, (306 b) connected by the pillars (308), and the plurality of thepillars (308) drawn from the one of the layers (306 a) pass through thedifferent spaces (310) in the at least one ply (312) between the two ofthe layers (306 a, 306 b).

3. The composite article (300 b) wherein the fiber tows (340 a, 340 b,340 c, 340 d) are disposed in a plurality of plies (336 a, 336 b,336 c,336 d) or layers (338 a, 338 b, 338 c, 338 d), the spaces (342) arethrough a plurality of the layers (338 a, 338 b, 338 c, 338 d) or plies(336 a, 336 b,336 c, 336 d), and the spaces (342) are bounded by fibertows (340 a, 340 b, 340 c, 340 d) in different layers (338 a, 338 b, 338c, 338 d) or plies (336 a, 336 b,336 c, 336 d). The differentorientations (370) of the fiber tows (340 a, 340 b, 340 c, 340 d) createthe spaces (310) between the fiber tows (340 a, 340 b, 340 c, 340 d).

4. The composite article (300, 300 b. 300 c) wherein the pillars (308)and/or the layers (306 a, 306 b) comprise material (314) used foradditive manufacturing.

5. The composite article (300, 300 b, 300 c) wherein the layers (306 a,306 b) and/or the pillars comprise a thermoplastic or a hybrid of thethermoplastic.

6. The composite article (300, 300 b, 300 c) wherein the pillars (308)and/or the layers comprise a thermoplastic comprising polyamide,polyetherketone (PEK), polyetheretherketone (PEEK),polyetherketoneketone (PEKK), polyimide (PI), or polyetherimide (PEI)

7. The composite article (300, 300 b, 300 c) wherein the fiber tows(302) comprise at least one material (314) chosen from fiberglass,kevlar, thermoplastic, and carbon.

8. The composite article (300, 300 b, 300 c) wherein the pillars (308)and/or the layers comprise a hybrid of a thermoplastic including atleast one additive or inclusion selected from a surfactant, astabilizer, a powder, a fiber, and a particulate.

9. The composite article (300, 300 b, 300 c) wherein the fiber tows(302) each have a diameter D of at least 2 mm and include at least 1000fibers (referring to FIGS. 3, 7A, 9).

10. The composite article (300, 300 b, 300 c), wherein the plies (312)each have a thickness T3 in a range of 2-10 mm (referring to FIGS. 3,7A, 9).

11. The composite article (300, 300 b, 300 c) wherein the pillars (308)and the layers (306 a, 306 b) each independently have a thickness T2 ina range of 2-5 mm (referring to FIGS. 3, 7A, 9).

12. The composite article (300, 300 b, 300 c) wherein the pillars (308)each have a L length in a range of 1-3 mm (referring to FIGS. 3, 7A, 9).

13. The composite article (300, 300 b, 300 c) having a total thickness Tin a range of 1.0 mm-50 mm (referring to FIGS. 3, 7A, 9).

14. The composite article (300, 300 b, 300 c) wherein the plies (312)comprising the fiber tows (302) arranged into a braided fabric, a wovenfabric, a non-crimp fabric, or unidirectional tape.

15. The composite article (300 b) wherein the fiber tows (340 a, 340 b,340 c, 340 d) are arranged in braids including yarns and the pillars(308) pass through spaces (310) between the yarns or between the fibertows (340 a, 340 b, 340 c, 340 d).

16. The composite article (300, 300 a, 300 b) wherein the fiber tows(302) are arranged in braids including yarns and the pillars (308) passthrough spaces (310) between the yarns or between the fiber tows (302),and the yarns pass through spaces in between the spaces (310) in the ply(312).

17. The composite article (300, 300 a, 300 b) wherein the spaces (310)between the fiber tows (302) are in an intermediate layer between thefiber tows (302) and are made of thermoplastic.

18. The composite article (300, 300 b, 300 c) further comprising resinfilling gaps between the plies (312) and bonded to pillars (308), thelayers (306 a, 306 b), or the layers (306 a, 306 b) and the pillars(308).

19. The composite article (300, 300 b, 300 c) wherein the layers and/orthe pillars (308) have a roughened surface that aids mechanicalinterlocking with resin.

20. The composite article (300, 300 b, 300 c) wherein the pillars (308)are thicker at a base (318) from which the pillar is drawn from the oneof the layers (306 a).

21. The composite article (300, 300 b, 300 c) wherein the pillars (308)are inclined from the one of the layers (306 a) to the another of thelayers (306 b).

22. The composite article (300, 300 b, 300 c) wherein the extrusions orpillars (308) are distributed uniformly.

23. The composite article (300, 300 b, 300 c) wherein the extrusions orpillars (308) are distributed on-uniformly, for example as a function ofthe need for increased pull off strength. In one or more examples, thecellular network (704) is concentrated near a perimeter of the compositefor 3 mode improvement.

24. A joint (1002) comprising the composite article (300, 300 b, 300 c),the joint comprising a skin (1004), a stiffener (1006), and aninterfacial region (1008) between the skin (1004) and the stiffener(1006), wherein the interfacial region (1008) comprises a compositearticle (300, 300 b) including fiber tows (302), and a network (304)comprising material (314) drawn between the fiber tows (302) and forminga physical barrier reducing propagation of cracks in the compositearticle. The interfacial region (1008) comprises a portion of the skin(1004), a portion of the stiffener (1006), and/or a layer between theskin (1004) and the stiffener (1006).

25. In one or more variations, the pillars (308) comprise or arereplaced by struts, walls, extrusions, or supports that pass through theplies (e.g., fabric).

b. Pillar Morphology

The morphology of the deposited material (314) can be controlled usingthe 3D printer (400). In illustrative embodiments described herein, thekey elements of the deposited material (314) include the anchor (316),the pillar or extrusion base (318) and the upright (320) or extrusion,as illustrated in FIG. 3B.

TABLE 1 Example extrusion and anchor morphologies as a function of printsettings Component name Print Setting Dimensions rangeDescription/Comments Anchor (316) Nozzle extruding Length: >1 mmTypically 3 mm long and 0.43 mm Width: 0.15-0.6 mm wide for a 0.4 mmnozzle Width and shape vary depending on where on the carbon fibre matthe anchor crosses. Extrusion Nozzle extruding Length: 0.3-1.5 mm longTypically 0.8 mm long base (318) for ~0.5 mm and (depending oncombination Coned area with base to tip off for the remainder of userprogramming and ratio ~3.5 of the process. nozzle diameter and/or speed)Extrusion or Nozzle head not Length: 2.5-5 mm (dependent Typically 2.5mm long with average upright (320) extruding on average diameter ofdiameter of 0.01 mm for a 0.4 mm 0.002-0.03 mm. nozzle Coning ratio withbottom to top of extrusion ~1.25

Note: The dimensions ranges are greatly determined by the nozzle (402)diameter, print settings and the user input (software/programminglanguage). In any of the examples described herein, the pillars (308)may be defined as comprising the upright (320) or extrusion only (notincluding the base (318)). In other examples, the pillars (308) aredefined as comprising the extrusion base (318) and the upright (320) onthe extrusion base (318). The height H and average diameter Daverage mayrefer to the combined height of the base (318) and upright (320) or theheight of the upright (320), for example.

In one or more examples, the layers (306 a, 306 b) and/or the pillars(308) have a roughened or irregular surface that aids mechanicalinterlocking with the resin.

In one or more examples, the pillars (308) are thicker at a base (318)from which the pillar (308) is drawn.

c. Preforming and Post Processing Techniques

FIGS. 8A-8D illustrate a method of forming a composite article usingvacuum forming and resin infusion.

FIG. 8A illustrates a first step of 3D printing on/through each of aplurality of plies (312) comprising mats (800) to form one or more mats(800) with printed architectures (e.g., comprising pillars (308) asdescribed herein). In one or more examples, the mats (800) comprisecarbon fiber mats. Although FIG. 8A is described for the case where theplies include mats (800), method illustrated in FIGS. 8A-8D can also beimplemented using any ply (including any of the example plies describedherein).

FIG. 8B illustrates a second step of stacking the mats (800) with theprinted architectures, thereby forming a stack (802) of the mats (800).The mats (800) comprising a plurality of fiber tows (302) are placed sothat each of a plurality of the pillars (308) pass through a spacebetween the fiber tows (302). In one or more examples, the extrusions orpillars (308) that go through the pores or spaces (310) can then linkwith the material (314) that is printed on the overlying printed mat(800).

FIG. 8C illustrates a third step of vacuum forming, comprising placingthe stack (802) under vacuum (804) at a temperature near the material'smelting point (in the example where the material (314) is athermoplastic, at the temperature near the thermoplastic melting point),for example 170° C. for nylon 12. In one or more examples, the stack(802) of mats (800) are preformed by placing the mats (800) in anenclosure (806) and a vacuum bag (808) under the vacuum (804) at atemperature just above the melting temperature of the material (314). Inone or more examples, the vacuum forming also enables more linkingbetween the mats (800) and the network (304) (e.g., cellular network704).

FIG. 8D illustrates infusing the stacked and vacuum formed mats (800)using a resin infusion method. In one or more examples, the mats (800)are infused by resin in a liquid molding process. The resin fills gapsbetween the mats (800) and is bonded to pillars (308) and/or the layers(306 a, 306 b).

FIG. 9 illustrates an alternative to vacuum bag preforming, comprisingusing a hot rolling iron 900 to apply pressure and heat to the stackedplies (312) or mats (800) along the direction (902) shown, so as toencourage the material (314) (e.g., thermoplastic) to go through thefiber layers (i.e., between the fiber tows 302) in the plies (312). Theiron (900) could be used at the end of a single layup of a ply (312) orafter the layup of a given amount of plies (312).

Example Applications

In one or more embodiments, the cellular networks described herein areused to toughen composites used on aircraft, particularly at high stressconcentration regions where mode I or mode II loads are experienced,e.g., in an Integrated Aircraft Structure (IAS). Integrated AircraftStructures are currently being joined by fasteners that provide a sourceof redundancy to encourage predictable failure in an otherwise brittleepoxy due to the poor fracture toughness properties of composites.However, the primary problem with using fasteners is the increasedweight they induce as parts are often made thicker than necessary toaccount for the high stress concentrations from the fastener holes.Additionally, composite failure due to bolts and fasteners initiateslocally at the hole but then tends to propagate in the through-thicknessdirection. By increasing the fracture toughness and providing a meansfor a more predictable failure, IAS can be joined more effectively usingcomposite articles described herein. More specifically, embodiments ofthe present invention improve mode I and mode II fracture toughness toprovide a way for stable composite failure needs to be employed so as toeither reduce the fasteners used for connecting composite parts, or tominimize the safety factor that is applied to part thickness when beingmechanically joined.

FIG. 10 illustrates an integrated aircraft structure 1000 (a T-joint1002) comprising a skin 1004, a stiffener 1006, and an interfacialregion 1008 between the skin (1004) and the stiffener 1006. Theinterfacial region (1008) comprises a composite article (300) asdescribed herein including fiber tows (302) and a network (304)comprising material (314) drawn between the fiber tows (302) and forminga physical barrier reducing propagation of cracks in the compositearticle (300). The interfacial region (1008) may comprise a portion ofthe skin (1004), a portion of the stiffener (1006), and/or a layerbetween the skin (1004) and the stiffener (1006).

The composite articles (300) according to embodiments of the presentinvention are not limited to use in integrated aircraft structures. Thecomposite articles described herein may be used in any applications thatrequire improvement in the damage tolerance of composites.

Advantages and Improvements

The present disclosure describes a continuous through thickness methodthat can provide the surprising and unexpected combination of improvedfracture toughness and damage tolerance, without degradation of thein-plane properties, as well as significant weight savings when used inaircraft applications (consequently lowering the cost of air travel).

There is currently no known composite toughening technique or structuresimilar to the composite articles and techniques described herein. Thethermoplastic network combined with the plies according to embodimentsillustrated herein is different from conventional veils because it is 3Dthroughout the thickness of the composite. Moreover, exemplarythermoplastic networks also differ from conventional through thicknesstechniques as microstructural defects to the fibers in the fiber towsare eliminated or comparatively suppressed. In addition, conventionalthrough thickness techniques to do not enable thermoplastic to linkaround the carbon tows so as to form a cellular network. However,illustrative composite articles described herein have the generalcharacteristics of a veil with the added feature that a first veil likestructure is now connected through the pores of the woven fabric toanother veil.

Processing Environment

FIG. 11 illustrates an exemplary system 1100 used to implementprocessing elements needed to control the 3D printers (400) describedherein.

The computer 1102 comprises a processor 1104 (general purpose processor1104A and special purpose processor 1104B) and a memory, such as randomaccess memory (RAM) 1106. Generally, the computer 1102 operates undercontrol of an operating system 1108 stored in the memory 1106, andinterfaces with the user/other computers to accept inputs and commands(e.g., analog or digital signals) and to present results through aninput/output (I/O) module 1110. The computer program application 1112accesses and manipulates data stored in the memory 1106 of the computer1102. The operating system 1108 and the computer program 1112 arecomprised of instructions which, when read and executed by the computer1102, cause the computer 1102 to perform the operations hereindescribed. In one embodiment, instructions implementing the operatingsystem 1108 and the computer program 1112 are tangibly embodied in thememory 1106, thereby making one or more computer program products orarticles of manufacture capable of performing the printing methodsdescribed herein (e.g., as described in FIG. 5). As such, the terms“article of manufacture,” “program storage device” and “computer programproduct” as used herein are intended to encompass a computer programaccessible from any computer readable device or media.

Those skilled in the art will recognize many modifications may be madeto this configuration without departing from the scope of the presentdisclosure. For example, those skilled in the art will recognize thatany combination of the above components, or any number of differentcomponents, peripherals, and other devices, may be used.

CONCLUSION

This concludes the description of the preferred embodiments of thepresent disclosure. The foregoing description of the preferredembodiment has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit thedisclosure to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of rights be limited not by this detailed description,but rather by the claims appended hereto.

What is claimed is:
 1. A composite article, comprising: a plurality offiber tows; and a network comprising layers connected by pillars,wherein: each of a plurality of the pillars are drawn from one of thelayers and pass through a different space between the fiber tows so asto connect the one of the layers to another of the layers, and thenetwork forms a physical barrier reducing propagation of cracks in thecomposite article.
 2. The composite article of claim 1, furthercomprising the fiber tows disposed in a plurality of plies, wherein: atleast one of the plies includes a plurality of the different spaces, theat least one of the plies is between two of the layers connected by thepillars, and the plurality of the pillars drawn from the one of thelayers pass through the plurality of the different spaces in the atleast one of the plies between the two of the layers.
 3. The compositearticle of claim 1, wherein the fiber tows are disposed in a pluralityof plies, the different spaces are through the plies, and the differentspaces are bounded by the fiber tows in different ones of the plies. 4.The composite article of claim 1, wherein the layers, the pillars, orthe layers and the pillars comprise material used for additivemanufacturing.
 5. The composite article of claim 1, wherein: the layers,the pillars, or the layers and the pillars comprise a thermoplastic or ahybrid of the thermoplastic, the thermoplastic comprises at least onepolymer chosen from a polyamide, a polyetherimide, a polyetherketone, apolyetheretherketone, a polyetherketoneketone, polyimide, and apolyphenylsulphone, and the fiber tows comprise at least one materialchosen from fiberglass, kevlar, and carbon.
 6. The composite article ofclaim 5, wherein the hybrid of the thermoplastic includes at least oneadditive or inclusion selected from a surfactant, a stabilizer, apowder, a fiber, and a particulate.
 7. The composite article of claim 2,wherein: the fiber tows each have a diameter of at least 2 mm andinclude at least 1000 fibers, the plies each have a thickness in a rangeof 2-10 mm, the layers each have a thickness in a range of 2-5 mm, thepillars each have a length in a range of 1-3 mm, and the compositearticle has a total thickness in a range of 1 mm-50-mm.
 8. The compositearticle of claim 2, wherein the plies comprise the fiber tows arrangedinto a braided fabric, a woven fabric, a non-crimp fabric, or aunidirectional tape.
 9. The composite article of claim 2, furthercomprising a resin filling gaps between the plies and bonded to pillars,the layers, or the layers and the pillars.
 10. The composite article ofclaim 9, wherein the layers, the pillars, or the pillars and the layershave an irregular surface that aids mechanical interlocking with theresin.
 11. The composite article of claim 1, wherein: the pillars arethicker at a base from which the pillars are drawn from the one of thelayers, and the pillars are inclined from the one of the layers to theanother of the layers.
 12. The composite article of claim 1, furthercomprising resin in the different spaces.
 13. A composite article,comprising: a network comprising a first layer and a second layerconnected by a plurality of pillars; and a plurality of fiber towsbetween the first layer and the second layer, wherein: each of theplurality of the pillars are drawn from the first layer and pass betweenat least two of the fiber tows so as to connect the first layer to thesecond layer, and the network forms a physical barrier reducingpropagation of cracks in the composite article.
 14. A joint comprisingthe composite article of claim
 1. 15. An integrated aircraft structure,comprising: a skin, a stiffener, and an interfacial region between theskin and the stiffener, wherein: the interfacial region comprises acomposite article including: a plurality of fiber tows, and a networkcomprising layers connected by pillars, wherein: each of a plurality ofthe pillars are drawn from one of the layers and pass through adifferent space between the fiber tows so as to connect the one of thelayers to another of the layers, and the network forms a physicalbarrier reducing propagation of cracks in the composite article, and theinterfacial region comprises a portion of the skin, a portion of thestiffener, and/or an interfacial layer between the skin and thestiffener.
 16. A method of manufacturing a composite article,comprising: forming a network of layers connected by pillars andcoupling a plurality of plies, including: (a) depositing material froman outlet onto a base layer while moving the outlet and the base layerrelative to one another, first in an x-y plane and then in az-direction, so as to form an anchor on the base layer; (b) moving theoutlet and the base layer relative to one another with no feed of thematerial from the outlet, so that a portion of the anchor is drawn tocreate one of the pillars; (c) moving the outlet and the base layerrelative to one another so that the outlet is positioned above a nextlocation on the base layer; (d) repeating steps (a)-(c) so as to createa plurality of the pillars on the base layer; (e) providing one of theplies comprising a plurality of fiber tows so that each of the pluralityof the pillars pass through a different space between the fiber tows;and (f) coupling a layer to the plurality of the pillars so that theplurality of the pillars pass between the fiber tows before connectingwith the layer; and repeating steps (a)-(f) using the layer as the baselayer in the next step (a) so that a composite article comprising thepillars, a plurality of the layers, and the plurality of the plies ismade, the composite article comprising: the plurality of fiber tows; andthe network comprising the layers connected by the pillars, wherein:each of the plurality of the pillars are drawn from one of the layersand pass through the different spaces between the fiber tows so as toconnect the one of the layers to another of the layers, and the networkforms a physical barrier reducing propagation of cracks in the compositearticle.
 17. The method of claim 16, wherein the base layer comprises amat of the material deposited using three dimensional printing.
 18. Themethod of claim 16, wherein the pillars are manufactured using a threedimensional printing and the fiber tows and the layers are fabricatedusing one or more methods different from the three dimensional printing.19. The method of claim 16, wherein the pillars and the layers eachcomprising a mat are fabricated using fused deposition modeling.
 20. Themethod of claim 16, wherein the providing comprises placing the one ofthe plies after formation of the plurality of the pillars.
 21. Themethod of claim 16, wherein the providing comprises placing the one ofthe plies prior to formation of the plurality of the pillars so that ineach step (b) the portion of the anchor is drawn between two of thefiber tows to create the one of the pillars.
 22. The method of claim 16,wherein at least some of the pillars are bonded to at least one of thefiber tows as the portion of the anchor is drawn between two of thefiber tows in each step (b).
 23. The method of claim 16, furthercomprising performing a post-processing step wherein at least some ofthe pillars are bonded to at least one of the fiber tows after the fibertows and the at least some of the pillars have been deposited.